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Borrowed from Japaneseヤンデレ(yandere), a portmanteau of 病んでる(yanderu), contraction of 病んでいる(yandeiru), progressive tense of 病む(yamu, “to be sick”), and デレデレ(dere-dere, “to become lovey-dovey, infatuated, lovestruck”).[1][2]
Developed on the model of tsundere(“being cold and even hostile towards another person before gradually showing a warm and caring side over time”).

Looking at anime charts, there's always the harem series with the dense hero and a bunch of girls whose personalities are pulled out of a set cast of tropes (the Childhood Friend, Tsundere, Yandere, and Lolita, for instance).

“Believe it, man. In fact, she's seriously creepy. Like creepier than that pink-haired girl from Future Diary.”
“Wait, what?” Sam got that reference quickly, but was not happy with the comparison. She wasn't a psychopathic, murderous Yandere stalker, from what he could see.